Exploring Different CO2 Systems

The field of refrigerant regulation is constantly changing, with additional regulations being announced on a nearly annual basis. Whether as a way to fight against ozone depleting substances or in an attempt to slow climate change, global refrigeration treaties like the Montreal and Kyoto Protocols have been phasing out refrigerants since the 1980s. One refrigerant that is not at risk to be phased out is carbon dioxide or CO2.

CO2 is has the lowest possible GWP rating of 1 and poses no risk to the ozone layer, making it immune to future regulations. And while CO2 is a very efficient refrigerant, it has limitations, particularly with regard to high ambient temperatures. However, emerging technology may have finally overcome this shortcoming. This article is intended to serve as a review the different technologies related to CO2. To better establish the differences between each kind of system, we will review each one, starting with booster systems.

1.Booster systems:

Booster systems have been used in Europe since 2006 and there are around 10,000 in operation today. Due to organic market forces, the booster system is the standard CO2 system throughout the world. Starting with the high pressure in the MT suction line, the refrigerant moves into the receiver with an approximate 50/50 mix of liquid and gas and the gas is sent through the gas bypass valve to keep the receiver pressure low. The gas is then sent over to the MT compressors and the liquid is sent through the liquid line to the expansion valves. Dry expansion happens here so that the gas gets superheated before going back to the compressors.

Boosters have the advantage that they are very simple compared to other CO2systems currently available. And while booster systems have the longest track record of any CO2 system, they have mostly been used in cooler climates, most notably in Northern Europe, where they are more energy efficient than R-404A systems. However, booster systems may not be the best choice in places with warmer ambient temperatures as they are less efficient than systems that use R-404A. Boosters are used in systems of a wide range of sizes, from small to very large.

2.Parallel compression:

The next topic in our discussion is parallel compression. Roughly half of the compressor capacity on the MT side can be pulled out and some of that can be put back through parallel compression, with the actual number of compressors is reduced.
Parallel compression was the first real breakthrough in making CO2 a viable option for warmer areas, showing significant COP improvements in warm climates. Parallel compression also reduces the swept volume, lowering the installation cost. Contrasting booster systems, parallel compression systems are not appropriate for systems smaller than 100–150 kW as dividing smaller systems in two is difficult. While much more complex than booster systems, this is a well-tested system that currently has several hundred in operation.

To illustrate this, imagine 100% mass flow through a gas cooler which goes from the high pressure expansion device, such as a valve or an ejector. Instead of sending 45% of the gas through the gas bypass valve to the MT compressors, the gas will be taken care of by the parallel compressors. The rest of the gas will go into the evaporators, evaporate, and return to the MT compressors. In this case, the efficiency gain is from increasing the suction pressure from about 150 to 175 psi for 45% of the gas. Running at a higher suction pressure, the compressors run more efficiently. Additionally, running at a higher suction pressure allows the compressors to move higher capacities with the same swept volume, offering savings at a set volume.

3.Ejectors:

Ejectors are devices that use high pressure to compress a second flow of gas. Ejectors have been around for over a hundred years, though they are new to refrigeration—they were originally used in water pumps.

4. Parallel compression with gas ejector:

Taking parallel compression and adding an ejector adds additional advantages. Gas ejectors move the gas from MT suction to the parallel compressor, which makes CO2 viable for areas with high ambient temperatures. While these systems are more complex than simple booster or standard parallel compression systems, they use small compressors, which reduce their footprint and make them more flexible. In the coming years, we expect to see CO2 technology to continue to improve, surpassing the already impressive efficiency.